Reductionism vs Systems approach
This essay contrasts two frameworks for assuring the safety of complex engineered systems: systems thinking and reductionism. Reductionism attributes unsafe behaviour in such systems to individual component failures but overlooks interactions among interdependent constituents in complex systems. By contrast, for complex systems, the systems approach - grounded in the CESM metamodel - explicitly models emergent behaviour arising from interactions among interdependent constituents and the system's environment, enabling more adequate safety assurance. This more adequate safety assurance stems from the systems approach's use of multi-level abstraction, which organizes the system into hierarchical tiers, each analysed across the dimensions of Composition, Environment, Structure, and Mechanisms (CESM). By analysing emergent behaviour across these dimensions and tiers - up to system-wide interactions - this approach predicts unsafe outcomes arising from interdependencies, non-linear dynamics, and environmental factors, not just individual component failures. The essay argues that while reductionism suffices for simple, decomposable systems, the systems approach is imperative for complex systems, where its ability to model emergent behaviour and interdependencies provides rigorous grounds for well-justified confidence in the system's behaviour - a capability beyond the reach of reductionism.